In recent years, fiber-optic cables have been increasingly used for communications, particularly in telephone and cable TV systems. Currently it is possible to manufacture long, continuous strands of optical fiber, which may propagate signals without substantial attenuation over long distances. It is also possible to manufacture the fiber structure as an optical wave-guide wherein only preselected modes of light propagate in the fiber. By limiting wave propagation through the fiber to a single mode, the bandwidth of the optical fiber may be exceedingly high to provide a high information-transfer capacity without signal dispersion related problems., Moreover, optical-fiber transmission equipment is compact, lightweight, and potentially inexpensive. Transmission over optical fibers does not generate interference and is unaffected by external interference.
Typically, a long haul and/or high bandwidth signal transmission system employing fiber optics, includes a light source such as a laser diode or an LED, and a photo detector such as a photodiode, connected through a single mode fiber-optic or optical wave-guide cable. Information is typically transmitted in digital form, as a series of light pulses that form a bit stream.
While transmitting information over optical fibers or wave-guides has numerous advantages, information transmission through fibers and their component waveguides suffers from laser-light launching losses into single mode fibers and wave-guide channels whose cross sectional dimensions are in micron range. Typical coupling efficiencies are about 50%. This necessitates using higher power, and therefore cost laser sources and/or using a large number of expensive and cumbersome optical amplification systems including additional pump lasers, Erbium Doped fibers, couplers, gain flattener, optical filters, polarization controllers to compensate for the losses due to the low coupling efficiency.
The simplest coupling system involves bringing the output end of a radiation source in butting engagement with the input end of the receptor. The radiation source may be a laser, an output end of a single mode fiber, a waveguide output etc. Butt coupling suffers considerably from the fiber core-cladding eccentricity and is effective only in permanent junctions. The more customary coupling method involving focusing the output of the radiation source, typically a laser, onto the input of the receptor fiber using a focusing lens is limited in that the focused radiation spot is diffraction limited. In practice the minimum spot size that can be achieved due to the difficulty in obtaining an ideal Gaussian spot is larger than the diffraction limited spot. When such coupling is employed to couple a laser source to a single; mode fiber having typical core diameter of 3–9 microns, the coupling efficiency drops to about 55%.
A method proposed by Peschel et al. using prism coupling is suitable only for large fiber bending angle configurations and re,quires the stripping of the fiber protections and cladding. Fiber stripping is also shown in U.S. Pat. No. 4,784,454 (Dyott) and U.S. Pat. No. 4,991,922 (Dahlgren) which disclose fiber optic coupling methods.
It has also been shown that the use of an interferometer can enhance the coupling efficiency in aquasi-phase-matched second harmonic generation process in a 4 μm wide titanium phosphate waveguide by as much as 61%. (Effects of interference in quasiphase-matched periodically segmented potassium titanyl phosphate waveguides, Zachary S. Benaich et al. Applied physics letters, Volume 75, Number 21, Nov. 22, 1999, incorporated herein by reference). The disclosed technique involves passing the fundamental beam through half waveplates and beam splitter cube combination that allows the variation of the power ratio of the two beams and individually coupling each beam into the wave guide using a lens. While this method may be implemented in a laboratory, it suffers in that it is extremely sensitive to vibration and therefore impractical for commercial applications.
There is thus still a need for an efficient coupler for coupling a radiation source to the input of a receptor single mode fiber or optical wave-guide, that is practical, reliable and easy to implement.